Traditionally, PIN (p-type-intrinsic-n-type) diodes are fabricated by the growth, deposition, or other placement of layers vertically on a substrate.
A PIN diode usually comprises a N-type doped substrate on top of which is deposited an intrinsic material layer followed by a P-type doped layer.
The top, P-type region is the anode and the bottom, N-type region or substrate is the cathode. When unbiased, the PIN diode is in a high impedance state and can be represented as a capacitor the capacitance of which is given byC=AAnodeDsiEo/T where:AAnode is the area of Anode;Dsi is the dielelectric constant of the intrinsic silicon;Eo is the permittivity of free space; andT is the distance between the anode and cathode.
If a positive voltage is applied to the anode with respect to the cathode that is larger than a threshold value, a current will flow through the diode and the impedance will decrease. A PIN diode in a forward biased state can be represented as a resistor whose value decreases to a minimum value as the current through the PIN diode increases. The bias to change the PIN diode from the high impedance (off) state to the low impedance (on) state can be DC or AC. In the case of an AC voltage, the magnitude must be greater than the threshold value and the duration of the positive voltage must be longer than the transit time of carriers across the intrinsic region.
The higher the power of the RF energy and/or the lower the frequency of the energy, the more readily a PIN diode will turn on. Thus, certain combinations of voltage and RF frequency will cause the junction between the P-type region and the intrinsic region to fill with carriers and turn on the diode.
This property of PIN diodes has led to their use as passive limiters to protect other devices in microwave and other RF applications. For instance, in a radar that both transmits and receives, a low noise amplifier may be coupled to the antenna to amplify received signals. The receive circuitry may be configured to be extremely sensitive in order to pick up weak radar signals from great distances. Amplifiers and other circuitry have limited dynamic range. Thus, inherently, if a low noise amplifier and surrounding receive circuitry is particularly adapted to be extremely sensitive so that it can pick up very weak signals, it typically will not be able to handle large signals and thus may be damaged if exposed to a very powerful signal, such as may be coupled to the receiver input by reflection from the antenna during transmit periods or otherwise.
In such cases, it is desirable to place a limiting circuit between the antenna and the low noise amplifier to protect the amplifier from overload. For instance, it is known to place PIN diodes in shunt with a circuit in a microwave application in order to protect that circuit from being overloaded and damaged by signals exceeding the power handling capabilities of that circuit. Particularly, if the input signal is relatively small, the PIN diode essentially behaves as a small capacitor and has little impact on the operation of the circuit it is protecting. However, if the RF signal becomes relatively large, then the PIN diode starts to conduct and, therefore, behaves essentially as a resistor that shunts most of the signal to ground.
Given their properties as described above, PIN diodes are well-suited to be used for such power limiting or protecting functions in RF applications.
As an illustrative example, the circuit of FIG. 1A may be placed in between the antenna and the low noise amplifier in a radar system. A simple, passive receiver-protection limiter comprises a PIN (positive-intrinsic-negative) diode 102 and an RF choke inductor 103, both of which are in shunt with the main signal path between the input 104 (e.g., coupled to the antenna) and the output 105 (e.g., coupled to the receiver). In most limiter circuits, the input and output of the circuit include dc blocking capacitors 106, 107. A single-stage limiter can typically reduce the amplitude of a large input signal by 20 to 30 dB.
With reference to FIGS. 2A and 2B, which are a cross-sectional, elevation view and a top, plan view, respectively, a typical PIN diode 1 has a mesa-like cross sectional shape, as illustrated in FIG. 2A, and comprises a N-type substrate 4 forming the cathode of the diode, an intrinsic region 5, and a P-type anode region 6. An insulating layer 8 covers the entire mesa-like structure except for the top of the anode and the bottom of the cathode, each of which will need to be covered with a metal contact pad for purposes of electrical contact with other circuitry on or off chip. For sake of clarity, the Figures illustrate only the semiconductor aspects of the diode and omit the contact pads, and/or other metallizations. The cross sectional area of the diode in FIG. 2A decreases from the N layer 4 to the P layer 6. This produces a shape resembling a top-truncated frustrum of a cone (i.e., the mesa-like shape). The whole structure may be encapsulated in glass 9 or another suitable encapsulation material.
A detailed discussion of the use of PIN diodes as power limiters and the structures and properties of such diodes that dictate their performance in such applications can be found in Cory, R., “PIN-limiter diodes effectively protect receivers”, EDN, Dec. 17, 2004, which is incorporated herein by reference.
However, in short, there are essentially two aspects of the design of a PIN diode that most significantly dictate the power level and/or frequency at which the diode will turn on in such situations. They are the thickness, y, of the intrinsic layer 5 between the P layer 6 and the N layer 4, and (2) the area of the junction 7 between the P type anode and the intrinsic region. More particularly, the thinner the intrinsic region, y, the higher the capacitance and the smaller the duration of the positive going cycle above the threshold value necessary to turn on the diode. Thus, essentially, as the intrinsic region 5 decreases in thickness, the capacitance increases. However, the total capacitance should be kept within a certain range for purposes of impedance matching with the other circuitry in connection with which it is used. Also, the thinner the intrinsic region 5, the larger the capacitance per unit area. Thus, for a given thickness of the intrinsic region 5, designers can keep the capacitance within a useful range by decreasing the area of the P/I junction 7. However, the downside of decreasing the area of the P/I junction is that the thermal impedance of the device will increase, thereby decreasing the amount of power that the diode can handle without failure, i.e. the amount of energy that it can dissipate.
Accordingly, there are many trade-offs between all of the dimensions of the various regions of a PIN diode that a designer can use to obtain the performance desired for a particular application of a PIN diode. More specifically, in the case of designing PIN diodes for use as RF power limiters, the designer must balance the minimum power level that will turn the diode on so as to start dissipating power, on the one hand, and the maximum power level that it can handle and dissipate before failure. In many cases, the necessary compromise cannot be accomplished within a single PIN diode.
Therefore, it often is necessary to use two PIN diodes, ¼ wavelength apart from each other, as illustrated in FIG. 1B, which shows a limiter circuit similar to that of FIG. 1A, including DC blocking capacitors 116 and 117, an RF choke inductor 113, but employing two PIN diodes 112a and 112b between the input 114 and output 115 spaced ¼ wavelength apart from each other along the signal path. Diode 112b has a thinner intrinsic region so that it will turn on at a relatively lower power. Diode 112a has a thicker intrinsic region so that it can handle more power. Thus, when the power exceeds a first threshold, diode 112b will turn on and start dissipating and reflecting powerback towards the input. Then, as the power level continues to increase, diode 112a turns on and dissipates and reflects most of the power, thereby protecting, not only the circuit that is being protected, but also diode 112b. 
U.S. Pat. No. 5,343,070 discloses a PIN diode and a method for fabricating the same.